This project is oriented toward evaluating the mechanistic role for cellular membrane potentials as a part of the driving force for intestinal Na ion-dependent transport systems. Work for the current project period is aimed at development of methods for obtaining an accurate measurement of Na:sugar and Na:amino acid coupling stoichiometry. Earlier attempts at determining this relationship were in error because of failure to recognize that Na ion-dependent transport induces disturbances in membrane potential which in turn modifies the magnitude of basal and sugar-dependent Na ion fluxes. Work is underway to determine the relationship between changes in membrane potential and consequent changes in Na ion flux so that appropriate corrections in coupling stoichiometry can be accomplished. Another approach involves measuring the stoichiometry under conditions in which the membrane potential is experimentally clamped at a fixed value so that flux stoichiometry can be measured directly. Work is also continuing with regard to the use of carbocyanine dyes and hydrophobic cations for non-invasive measurement of cellular membrane potentials.